1. Field of the Invention
This invention is related to the field of video content protection.
2. Description of the Related Art
Computing devices, from desktop computers to mobile personal digital assistants and cell phone devices, have been provided with high quality video display devices and corresponding processing power that has made such devices a desirable platform on which to view video content. However, given the ease with which digital data can be duplicated by virtually any user, the problem of content protection (e.g. protection of the content creator's copyright and/or trademark rights) has presented itself. While there are a variety of digital rights management schemes, typically the schemes involve encrypting the video stream so that only an authorized user (having the key or keys needed to decrypt the video stream) can view the video. In order to save space, the video stream is typically compressed prior to being encrypted, using any of a variety of compression schemes such as those specified by the Moving Pictures Expert Group (MPEG). While a user can make a copy of the encrypted video and give it to another user, the other user generally will not have the correct keys and cannot view the video. The other user is then forced to obtain his or her own licensed copy of the video.
Once the video stream has been decrypted on a computing device, it is important to protect the decrypted video stream from capture by an unauthorized person (e.g. via intrusive monitoring such as connecting a logic analyzer to a bus or other electronic transmission medium in the device). Typically, manufacturers of decryption/decompression hardware and/or software are required, as a condition of licensing the ability to play protected video content, to ensure that the decrypted video (or “plaintext” video) is not accessible to any third party. Transmitting the plaintext video over an external connection from a chip to, e.g., a memory would violate the condition. However, because the decompressed video can be very large for high resolution video (and because decompression often requires reference to data in previous frames of the video stream as specified in motion compensation vectors embedded in the compressed video stream), it is a practical impossibility in current chips to retain all of the plaintext video information within a chip. For example, retaining enough frames of a 1080p resolution video to perform decompression (e.g. about 6 frames) may require on the order of 36 Megabytes, which is generally not possible to implement on chip with logic to perform the decompression/decryption. Currently, content creators permit an exception to the condition above, permitting some video data to appear in plaintext in memory. It is expected that content creators will eventually demand that the exception be repealed.